Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 93

4.4.4 Color Systems

There are many theories of how we perceive color (Sekuler and Blake 2005 ).
There are a lot of technical details and tools available which allow us to make
quite strong predictions about color perception. For our purposes, the most
important thing to remember is the high level distinction between the two ways
that we perceive color: additive and subtractive.
Computer displays use projected light (for the most part) and printed paper uses
reflected light. Projected light uses a different set of primary colors, the additive
colors of red, green, and blue (often referred to as RGB), while reflected light use
the subtractive colors: cyan, magenta, and yellow (with black as the key color,
which is why they are often referred to as CMYK). The additive colors start from
black, no light, and then add colors to get other colors, and then, when all are
added, white is produced. The subtractive colors remove colors from white,
effectively reducing the amount of light that is reflected, and ending up at black.
Thus, with current technology it is hard to get a pure black color on a display
screen (at least some of the lights tend to stay on), although it is possible to get
closer to pure black using subtractive colors.
The colors from printed materials cannot completely match the colors from a
screen because of the way the two different color systems work. There are some
tools available that can help you make the two match more closely, however, but
in the end the choice of how closely you make them match may be down to
personal preference.

4.4.5 Flicker

People are sensitive to flickering lights. The flicker can be made imperceptible,
however, by using rates that exceed the flicker fusion rate. In the early days of the
movies, cinema films ran at a frame rate below the flicker fusion rate, which is why
any movements appear jerky and you can still detect the flicker (which explains
why they were often referred to as the flicks). In modern cinema, the flicker
problems is overcome by typically showing films at 24 frames per second, with the
image being presented twice per frame on two-blade shutter projectors, and three
times per frame on three-blade shutter projectors.
It should be noted, however, that lights which flicker at a rate of 7-10 cycles per
second 1 can trigger epileptic fits in some people. Series 500 of the ISO 9241
standard on ergonomics of human-computer interaction is a guideline that
addresses this issue for display screen equipment, and you sometimes hear
warnings about flickering lights and images before they appear during a television

1

You will also see this measure in Hertz: 1 Hz is 1 cycle per second.